This invention relates to a microelectronic device of the metal/insulator/metal' (MIM') type, and in particular to such a device wherein the insulator is an organic material.
There has been considerable interest in recent years concerning the possible use of organic and polymeric materials in microelectronic devices where one of the dimensions of the device would be a dimension of a single molecule. Such "molecular electronic devices" (MED) would ultimately have the potential to supersede current microelectronic devices based on silicon or GaAs where the dimensions of elements are typically of the order of one micron (10,000 .ANG.) at present. The dimension of a MED would typically be the length of the specific molecule, about 50 .ANG.. Thin ordered monolayers of such dimension are typically prepared by Langmuir-Blodgett (LB) techniques. Preparation of LB monolayer films are described in the literature. For example, in Insoluble Monolayers at Liquid-Gas Interfaces (Interscience Publishers, New York, 1966), incorporated herein by reference, G. L. Gaines, Jr. describes such techniques.
A. Aviram and M. A. Ratner (Chem. Phys. Lett. 29, 277 (1974)) advanced a theory concerning the achievement of molecular rectification utilizing organic molecular structures of the D-.sigma.-A form. Aviram and Ratner theorized that unimolecular oriented films of organic molecules D-.sigma.-A where D is a strong electron donor (e.g. a tetrathiafulvalene, or TTF, moiety), .sigma. is a saturated covalent bridge and A is a strong electron acceptor (e.g. a tetracyanoquinodimethan, or TCNQ, moiety) when sandwiched between two metal films M.sub.1 and M.sub.2 (or M and M') could be a molecular rectifier.
Since that time there have been many references in the literature to the original evaluations of Aviram and Ratner, with general researchers attempting to synthesize such A-.sigma.-D molecules and then to construct rectifying devices from the resulting molecules. Reports of possible rectification in organic films of molecular thickness in scanning tunneling microscope (STM) studies have appeared, and two of these claims have subsequently been retracted (referenced below).
More recently, attention has turned to monolayers of a D-.sigma.-A type molecule in which D is a substituted phenyl carbamate moiety and A is a substituted tetracyanoquinodimethan (TCNQ) moiety. An aliphatic bridge serves to electronically insulate the donor and acceptor groups on the molecule, and constitutes a barrier to prevent tunneling of electrons between the terminal moieties. Molecules of this type have been synthesized and characterized. However, attempts to fabricate metal/insulator/-metal' (MIM') electronic devices from such molecules have heretofore been unsuccessful. For example, R. M. Metzger et al. reported (Synthetic Metals 28. C807, at C812-13 (1989)) the achievement of rectification in a device utilizing gold as the lower electrode and the bis-(dodecyl)aminophenylcarbamate of 2-bromo-5-hydroxyethoxy-TCNQ as the insulator, and with the nanotip of a modified scanning tunnelling microscope (SCM) acting as the top electrode; but then retracted the claim (R. M. Metzger et al (1989), supra, page C813, Note added in proof). A. Aviram et al. earlier reported (Chem. Phys. Letters 146, 490 (1988)) molecular switching and rectification in a similar device utilizing a hemiquinone as the insulating layer, as cited in Metzger et al. ((1989) supra). This claim also has been retracted (A. Aviram et al., Chem. Phys. Letters 162, 416 (1989)).
N. J. Geddes et al, in reporting the fabrication of MIM' structures in which the insulator is a fatty acid material applied to the substrate metal layer as a LB film (Proceedings of the Third International Symposium on Molecular Electronic Devices. F. Carter et al., eds., North Holland Publishers, Amsterdam, 1988, p. 495 ff) have described techniques that avoid damage to the fatty acid layer during deposition of a top electrode layer in a MIM' device. These techniques have facilitated the investigation of the electrical properties of a LB bilayer of such fatty acid materials (Geddes et al., Thin Solid Films 168. 151 (1989)).
Rectification has also been reported in the literature with reference to thin films of other organic and polymeric materials. However, the devices differ markedly from the MIM' devices described hereinbelow. For example, when an organic layer 20,000 .ANG. thick of zinc phthalocyanine (PcZn) utilized in a gold-PcZn-metal' device was exposed to air, a strong rectifying effect was noted (M. Martin et al., J. Appl. Phys. 54. 2792 (1983)).
The term "rectification" has been used by D. K. Smith et al. (J. Am. Chem. Soc. 108. 0522 (1986)) to describe a different mechanism, an electrically irreversible process in a redox polymer bound to an electrode in samples described as approximating a monolayer in thickness. As used herein, however, the term "rectification" is used to describe the converting of an alternating electrical current to a unidirectional current. The organic film, in this process, is a medium for electron transfer, in a single direction only, from one electrode to the other.
This application describes the synthesis of the p-dodecyloxyphenylcarbamate of 2-(2'-hydroxyethoxy)-5-bromo-7,7,8,8-tetracyanoquinodimethan (DDOP-C-BHTCNQ) as a molecular compound sufficiently pure to effectively act as the insulating layer in an MIM' device. It also describes the fabrication of a MIM' device in which the metal of the top layer differs from that of the substrate layer, utilizing either a single LB monolayer or an odd number of LB monolayers of this compound. The exemplary devices according to the invention described herein exhibit sufficient current-voltage (I/V) asymmetry to serve as a rectifying device. Such devices are useful as, for example, rectifiers, memory devices, switches, diodes, or transistors, and are activated by appropriate controls such as electric fields, light beams, heat, and the like.